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5.  Insoluble Substances. — 10-20 grains are dissolved in a little water,
the insoluble matter being filtered off, washed, dried, ignited and weighed.

6.  Alumina. — The filtrate from the preceding operation is heated with
a little ammonium chloride and ammonia quite; free from carbonate, the
precipitate being filtered off, washed, dried, ignited and weighed as AlaO.,
-f- Fc2O3-    The proportion of iron being known   from  determination  4,
that of the alumina may be calculated.

7.  Lime. — The filtrate from the alumina is precipitated with ammonium
oxalate, the calcium oxalate beingweighed, in the usual way as calcium oxide.

8.  Magnesia.— To the filtrate  from the preceding operation   (some-
what concentrated if necessary), ammonia and sodium phosphate are added;
after 24 hours the precipitate is filtered off, washed with slightly aimnouiaeal
water, dried, ignited, and weighed, as magnesium pyrophosphate ;  i part
of the latter = 0-36242 part of MgO.

9.  Quantitative Determination of the Sodium Sulphate,   -r gram
of the sulphate, dissolved in water, is treated with ammonia and ammonium
carbonate to precipitate1 alumina,, iron and lime,  the  liquid  being then
filtered and the filtrate evaporated to dryness with a few drops of pure sul-
phuric acid in a tared platinum dish ; the residue in ignited, at first alone
and later with a few crystals of pure ammonium carbonate., and weighed.
The. weight is diminished by that of the sodium sulphate corresponding
with the sodium chloride found (in 3) (i part NaCl      1-2 13(1 Na.jSO.j) and
by that of the magnesium sulphate corresponding with the magnesia found
(in 8)   (i part MgO ~ .-. 2-983(1 MgSO,,) ; the remainder gives the NaaS(.)4
present in i grain of substance.

Crystallised sodium .sulphate should contain ,|.|-i% Na,,SO(1 and 53-9%
IljjO, but usually it is somewhat eilloresced and the percentage of water rather
low; the commonest impurity is a, snia.ll amount of the chloride.

Tlu) crude anhydrous sulphate generally contains i -,.;% of moisture, its
free acridity being often above i% and the content of iron usually 0-0,5 U>i5%,
but sometimes 0-5% (0-15% is allowable) ; the proportions <>f insoluble matter,
sodium chloride, alumina, lime and man'iiesia, vary.

N!iaS -f ()1LO v..a.|.o

Deliquescent, colourless or more often greenish or yellowish crystals,
extremely soluble in water. The calcined (anhydrous) product is also sold
in grey or brown, irregular masses soluble in water. The must frequent
impurities arc : carbonaceous particles, ferric sulphide, sodium thiosulphate
and sulphate and free alkali (tests i 3); the content; of NaaS is found as in 4.

1,  Solubility.........5 grams should give a, clear solution in 50 c.c. of water.

If it does not do so (presence of carbon, ferric .sŤ//>/m/t'), the liquid is filtered
through a filter previously dried at 105" and tared and the insoluble matter
washed well with tepid water, dried at   105" and weighed.

2.  Thiosulphate, Sulphate.    2 grains are dissolved in a little water
and treated with excess of dilute hydrochloric acid, any turbidity indicatings of